The present invention relates generally to methods of forming articles made of polyolefin materials and, in a preferred embodiment thereof, more particularly provides a product produced by a method of reforming crosslinked polyethylene articles.
It is well known in the art to crosslink polyolefins. Such crosslinking is known to enhance certain physical and chemical properties of polyolefin materials. In particular, crosslinking has been shown to increase maximum useful temperature, reduce creep, improve chemical resistance, increase abrasion resistance, improve memory characteristics, improve impact resistance, increase resistance to oxidative degradation, and improve environmental stress crack resistance of polyolefin materials.
It is also well known in the art to subject polyethylene (PE) to a variety of crosslinking processes to produce crosslinked polyethylene (PEX). These PE crosslinking processes include addition of peroxide, addition of Azo compounds, electron beam irradiation, and addition of silane. The last of these utilizes silane to produce a grafted polymer.
Crosslinking of silane-grafted PE is described in a paper entitled xe2x80x9cPolymer Crosslinking with Union Carbide(R) Organofunctional Silane A-171xe2x80x9d published by Union Carbide Chemicals and Plastics Company, Inc. of Danbury, Connecticut, and in a paper entitled xe2x80x9cAdvances in Silane Crosslinking of Polyethylenexe2x80x9d by L. M. Panzer of Union Carbide Chemicals and Plastics Company, Inc., Tarrytown, New York, both of which are hereby incorporated by reference.
In one silane crosslinking process known as Sioplas, a PE resin is melted and silane is added to the melted PE along with a peroxide initiator. Crosslinking sites are thereby formed on PE polymer chains and crosslinking begins to occur. The grafted resin is pelletized and stored for later use in foil-lined bags. As crosslinking of the grafted resin occurs in the presence of moisture, it is important that the grafted resin not be exposed to moisture until substantial crosslinking is desired. However, since this method of crosslinking of PE is self-perpetuating, the crosslinking reaction producing moisture as a byproduct, it is not practical to completely prevent crosslinking in the pelletized grafted PE resin and so it has a shelf life of only approximately 6-9 months.
A catalyst masterbatch is prepared for the grafted resin. The catalyst masterbatch typically includes a quantity Of PE, a catalyst, an antioxidant, a stabilizer, and an internal lubricant. The catalyst masterbatch is typically pelletized for ease of mixing with the grafted resin in a conventional extruder.
The grafted resin and catalyst masterbatch are usually combined in a specific ratio, melted and mixed together, and extruded. When the grafted PE resin and catalyst masterbatch are mixed together, crosslinking of the PE at the silane graft sites accelerates. The material exits the extruder and is typically cooled in water.
In another silane crosslinking process known as Monosil, instead of preparing separate grafted PE resin and catalyst masterbatch, the PE resin, silane, peroxide initiator, catalyst, antioxidant, stabilizer, and internal lubricant are combined in one step. The materials are melted and mixed together, typically in the extruder, and extruded. A method of making crosslinked extruded products is described in U.S. Pat. No. 4,117,195, the disclosure of which is hereby incorporated by reference. AS with the Sioplas method, in the Monosil process the material exits the extruder and is cooled in water.
To complete the crosslinking process, the extruded material from either the Sioplas or Monosil process is generally exposed to moisture at an elevated temperature. A sauna-like environment or hot water immersion will produce the desired level of crosslinking in a relatively short period of time. When this curing stage is finished, the PE may be approximately 65-89% crosslinked.
PEX has been utilized for diverse applications, including wire and cable insulation, and pipe. Where pipe is made of PEX, the crosslinking process enhances its ability to contain fluid at elevated temperature and pressure. For this reason, commercial success has been achieved in utilizing PEX pipe in hot water heating applications and in potable hot water plumbing applications.
A disadvantage of PEX pipe, however, is that it is not suitable for forming sealing surfaces thereon. For example, riser tubes may not be conveniently formed from PEX pipe by heating the PEX pipe and forming the heated pipe between mating dies as described in U.S. patent application Ser. No. 08/622,061 entitled xe2x80x9cAutomated Tube Reforming Apparatus and Associated Methods of Manufacturingxe2x80x9d, the disclosure of which is hereby incorporated by reference. Since the material is substantially crosslinked, such reforming of the PEX pipe typically produces visible and objectionable interfaces, seams, and folds, which provide leak paths across the sealing surface formed thereon.
From the foregoing, it can be seen that it would be quite desirable to provide a product produced by a method of reforming crosslinked PE articles which does not produce visible and objectionable interfaces, seams, and folds on sealing surfaces of the articles, but which articles may be subsequently utilized in sealing applications, such as containment of pressurized hot water. It is accordingly an object of the present invention to provide such a product.
In carrying out the principles of the present invention, in accordance with an embodiment thereof, a method is provided which permits grafted polyolefin articles to be reformed with sealing surfaces thereon and subsequently crosslinked. The articles thus produced have the superior properties associated with crosslinked polyolefins, in combination with a desired functional shape, such as a hot water riser tube in plumbing applications having a radially enlarged sealing surface formed thereon.
In broad terms, a method of reforming a polymer material having a first shape as extruded from an extruder is provided. The method includes the steps of forming the extruded material so that the extruded material has a second shape, and then curing the extruded material. The polymer material may be a polyolefin, a silane-grafted polyolefin, and may be a silane-grafted polyethylene material.
In one aspect of the present invention, the forming step includes the steps of gauging a volume of the polymer material to be formed, heating the gauged volume of polymer material, forming the gauged volume of polymer material, and cooling the formed polymer material. The polymer material may be generally tubular-shaped prior to the forming step.
In another aspect of the present invention, the curing step includes subjecting the formed polymer material to moisture for a predetermined period of time. The formed material may also be heated while it is subjected to the moisture.
Also provided is a method of reforming and crosslinking a silane-grafted polyolefin article. The method includes the steps of heating the polyolefin article when it is approximately 1-60% crosslinked, reforming the heated polyolefin article between a pair of mating dies so that the heated polyolefin article is at least partially reshaped, cooling the reformed polyolefin article, and curing the reformed polyolefin article. The polyolefin article may be a tube made of silane-grafted polyethylene.
In one aspect of the present invention, the curing step includes subjecting the reformed polyolefin article to moisture until it is approximately 65-89% crosslinked. The curing step may be accomplished by placing the reformed polyolefin article in a steam chamber for a predetermined period of time.
Yet another method is provided by the present inventionxe2x80x94a method of forming a radially enlarged sealing surface on a tube made of a silane-grafted polyethylene material. The method includes the steps of gauging a volume of the tube to be formed, heating the volume of the tube to be formed, forming the heated volume of the tube into the radially enlarged sealing surface when the tube is approximately 1-60% crosslinked or, more preferably, approximately 1-40% crosslinked, cooling the formed volume of the tube, and further crosslinking the tube.
In one aspect of the present invention, the further crosslinking step includes exposing the tube to water until the tube is approximately 65-89% crosslinked. The tube may be further crosslinked by disposing the tube in an enclosure and introducing steam into the enclosure.
The present invention also provides products produced by the above methods.
The use of the disclosed method enables convenient reforming of articles made of polymer materials. Significantly, the method permits tubes made of silane-grafted polyethylene to be reformed so that radially enlarged sealing surfaces are formed thereon, without also causing visible and objectionable folds, seams, or interfaces to be formed on the sealing surfaces. Thereafter, the tubes may be cured so that they may be used in applications requiring a relatively large percentage of crosslinking.